Wireless communication equipment, such as cellular and other wireless telephones, wireless network (WiLAN) components, GPS receivers, mobile radios, pagers, etc., use multi-band antennas to transmit and receive wireless signals in multiple wireless communication frequency bands. Consequently, one of the critical components of wireless devices is the antenna which should meet the demands of high performance in terms of high signal transmission strength, good reception of weak signals, increased (or narrowed, if required) bandwidth, and small dimensions.
In mobile telecommunication, electromagnetic waves in the microwave region are used to transfer information. An essential part of telecommunication devices is thus the antenna, which enables the reception and the transmission of electromagnetic waves.
Cellular systems may operate in two different frequency bands called GSM (global system for mobile) and DCS (digital communication system). In Europe, the frequency bands for GSM 900, which are located at 880 MHz to 960 MHz, and GSM 1800 (DCS), located at 1710 MHz to 1880 MHz, are used. Additionally, there is the GSM 850 frequency band from 824 MHz to 894 MHz and the GSM 1900 (PCS) frequency band from 1850 MHz to 1990 MHz widely used in the United States.
Planar inverted F-antennas (PIFAs) have many advantages. They are easily fabricated, have a simple design, and cost little to manufacture. Currently, the PIFA is widely used in small communication devices, such as cellular phones. This is due to the PIFA's compact size that makes it easy to integrate into a device's housing, thereby providing a protected antenna. The PIFA also provides an additional advantage over, for example, the popular whip antennas with respect to radiation exposure. A whip antenna has an omnidirectional radiation field, whereas the PIFA has a relatively limited radiation field towards the user.
The PIFA is generally a λ/4 resonant structure and is implemented by short-circuiting the radiating element to the ground plane using a conductive wall, plate or post. Thus, the conventional PIFA structure consists of a conductive radiator or radiating element disposed parallel to a ground plane and is insulated from the ground plane by a dielectric material, typically air. This radiating element connects to two pins, typically disposed toward one end of the element, giving the appearance of an inverted letter “F” from the side view. The first pin electrically connects the radiating element to the ground plane, and the second pin provides the antenna feed. The frequency bandwidth, gain, and resonant frequency of the PIFA depend on the height, width, and depth of the conductive radiator element, and the distance between the first pin connected to the radiating element and ground, and the second pin connected to the antenna feed.
FIG. 2 illustrates a conventional PIFA 200 design. The conventional PIFA 200 includes a conductive plate which forms a radiating element 209 of the antenna. Radiating element 209 is disposed about parallel to a ground plane 210 formed on a substrate 211. This parallel orientation between radiating element 209 and ground plane 210 provides optimal performance, but other orientations are possible.
Radiating element 209 electrically connects to ground plane 210 via a tuning or shortening element 212, most often disposed at one side of radiating element 209 and a feed element 213. Feed element 213 is somewhat electrically insulated from ground plane 210. When electric current is fed to radiating element 209 mounted above ground plane 210 through feed element 213, radiating element 209 and ground plane 210 become excited and act as a radiating device.
The operating frequency or the resonance frequency of PIFA 200 can be modified either by adjusting the dimensions and shape of radiating element 209 or by moving the location of feed element 213 with respect to tuning element 212. The resonance frequency can also be finely adjusted by changing the height and/or width of tuning element 212. Thus, in the conventional PIFA, the operating frequency or resonance is fixed by the size, shape, or placement of feed element 213, tuning element 212, or radiating elements 209, respectively. To change the bandwidth of PIFA 200, the height must be increased which will lead to an undesirable increase in the overall antenna size. With a trend towards smaller terminals, i.e., thinner and shorter mobile terminals, with very limited space available for the antenna element (GSM/WCDMA), the bandwidth of the High Band, DCS, PCS, and UMTS (1710 MHz->2170 MHz) at −6 dB S11 is becoming more difficult to achieve.
PIFAs with parasitic elements are being used currently to enhance the High Band bandwidth, but usually use a flex film on the antenna carrier with an additional connection (c-clip or Pogo Pin) on the PCB.